1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to an automated method and apparatus for minimizing the optical path difference between the reference and the test arms of a Linnik white-light interference objective.
2. Description of the Related Art
White-light interferometric devices typically utilize microscope objectives for simultaneously imaging a sample surface and a reference surface onto a detector in order to produce interference fringes representative of the height of the sample. As illustrated in simple schematic form in FIG. 1, a typical Linnik interference microscope includes an objective 10 focused on a test surface S and incorporating an interferometer. The interferometer comprises a beam splitter 12 and a reference mirror R such that the light beam W directed to the sample surface S is split and also directed toward the reference mirror.
As is well understood by those skilled in the art, the light beams reflected from the reference mirror R and the test surface S (the reference and test beams, respectively) are combined to produce interference fringes as a result of the optical path difference (OPD) between the reference and test beams. The light reflected by the test surface S interferes with the light reflected at the reference mirror R and, according to the principle of superposition, bright interference fringes are produced corresponding to all points on the reference mirror where the optical path difference of the light is equal to a multiple of its wavelength. In order for discernible fringes to appear, the OPD must be within the coherence length of the light source. Spectral filtering may be used to increase the coherence length of the light.
The light from the reference and sample surfaces is typically passed back through the interferometric microscope 10 and appropriate imaging optics 14 toward an imaging array 16 positioned in a camera in coaxial alignment with the objective. The imaging array usually consists of individual charge-coupled-device (CCD) cells or other sensing apparatus adapted to record a two-dimensional array of signals corresponding to interference effects produced by the interferometer as a result of light reflected at individual x-y coordinates or pixels in the surface S and the reference mirror R and received at corresponding individual cells in the array. Appropriate electronic hardware (not shown) is also provided to process the signals generated by each cell and transmit them to a computer 18 for further processing. Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the array. The map may be displayed on a monitor 20 connected to the processing unit 18. As mentioned, a filter 32 may be provided to optionally narrow the bandwidth of the light beam W to a predetermined value.
In a Linnik interferometer, a smooth test surface S is typically profiled by phase-shifting interferometry (PSI), typically by scanning the reference arm of the microscope objective 10. A piezoelectric translator or equivalent device (not shown in FIG. 1) is used in conventional manner to shift the reference optics 30 and mirror R together with respect to the sample surface S. Phase shifting is founded on the basic concept of varying the phase difference between two interfering beams in some known manner, such as by changing the optical path difference in discrete steps or linearly with time. Under such conditions, three or more measurements of the light intensity at a pixel of a receiving sensor array can be used to determine the relative phase of the light reflected from the point on a test surface corresponding to that pixel, with respect to the other points on the test surface. Based on such measurements at each pixel of coordinates x and y, a phase distribution map "PHgr"(x,y) can be determined for the test surface, from which very accurate height data h(x,y) are calculated in relation to the wavelength xcex of the light source used by the following general equation:                               h          ⁡                      (                          x              ,              y                        )                          =                              λ                          4              ⁢                              xe2x80x83                            ⁢              π                                ⁢                      xe2x80x83                    ⁢                                    Φ              ⁡                              (                                  x                  ,                  y                                )                                      .                                              (        1        )            
Phase-shifting interferometry provides a vertical resolution in the order of {fraction (1/100)} of a wavelength or better. The technique is typically limited to measurements of smooth surfaces due to a height ambiguity encountered at phase steps greater than xcfx80.
Linnik interferometers are also used for vertical-scanning interferometry (VSI) of rough surfaces. VSI is a well-known technique where white light is used as the light source in an interferometer and the degree of modulation, or coherence, of interference fringes produced by the instrument is measured for various distances between the test surface and the reference surface of the interferometer (each corresponding to a different optical path difference) to determine surface height. The microscope objective is typically adapted for vertical movement (along the z coordinate). Thus, an interference-fringe map is generated by detecting the intensity of the light signal received in each cell of the light sensor as a function of the z-scan position. The position of the scanning mechanism corresponding to maximum interference at each pixel is determined and used, based on the distance from a reference point, to calculate the height of the surface at that pixel.
As shown in FIG. 1, the Linnik configuration of a microscope objective includes a white light source 22 and imaging optics 24 providing an illumination beam W to the system through a beam splitter or equivalent device 26. The illumination beam is then focused, ideally, in the entrance pupils E and Exe2x80x2 of the reference and test imaging optics 30 and 28, respectively. This is known as Kohler illumination, and produces approximately uniform illumination on the sample surface S and the reference surface R. By way of calibration, the reference mirror R is set at the focal point of the imaging optics 30 through a process based on a visual determination of best focus produced by axially shifting the reference mirror with respect to its imaging optics 30 along a distance F, as one skilled in the art would readily understand. This is often accomplished by the introduction of a field stop in the illuminator (not shown in FIG. 1). When the diameter of the field stop is reduced sufficiently to bring its edges into the field of view, a sharp image of the edges is formed on the camera when the reference surface R is in focus. Copending application Ser. No. 09/452,334 discloses a mechanism for focusing the reference mirror R automatically, which provides an opportunity for periodic recalibration of the instrument.
Prior to carrying out surface profiling by PSI measurements, the distance between the sample surface S and its focusing optics 28 is also adjusted to produce a sharp, in-focus image of the sample. This is normally achieved by moving the entire objective 10 vertically with respect to the sample stage (although the same result could obviously be achieved as well by moving the stage). The distance Fxe2x80x2 (FIG. 1) between the lens 28 and the surface S is varied, either manually or automatically, until the sample surface is in focus. Typically, the optimal in-focus position of the sample surface is determined by visual observation of the best picture of the sample surface seen at the monitor 20. Alternatively, an image of the field stop, as described above, can be used to set the optimal in-focus position. Similarly, for VSI measurements, fringes are present at a given scan position only for portions of the surface that are in focus.
When a light of short coherence length is used, such as produced by the white-light source 22, interference between the return beams only occurs when the optical path difference between the test and the reference paths is within the coherence length of the system. The coherence length of a white-light source can be increased with spectral filtering. Therefore, it is necessary to minimize the OPD in order to obtain satisfactory fringe contrast for PSI and VSI measurements. In general, for optimum performance the OPD between the reference and test paths must be less than approximately one wavelength. If a 40 nm bandwidth red light source is used, the coherence length is about 9 xcexcm and the OPD between the reference and test paths must be less than 0.6 xcexcm.
If a narrower bandwidth is chosen to carry out the interferometric measurements, such as with the use of a laser light, the coherence length and hence the OPD can be much greater, but stricter operational requirements render the commercial use of this type of interferometer less desirable for many applications. Therefore, short coherence lengths are preferred in many cases and the sample and reference path length difference must then necessarily be placed within very close tolerances. Minute changes in the optical path difference between the reference and test surfaces are sufficient to cause degradation of the fringe quality seen at the sensor 16.
Therefore, the quality of the phase-shifting and vertical-scanning measurements are also directly related to the degree to which the OPD is minimized.
A manual control is typically provided to adjust the length of the optical path of either the reference or the test beam. For example, a typical adjustment consists of manually translating the lens 30 and the reference mirror R together (thereby preserving the in-focus position of the reference mirror) with respect to the splitter 12 along a distance D (see FIG. 1), so as to generate the best visual fringe contrast. This adjustment is performed prior to performing measurements of the test sample S. In phase-shifting, measurements are accomplished using a scanning mechanism, typically a piezoelectric element, that also shifts the lens 30 and mirror R together in the spatial dimension of the distance D, herein also referred to as the D direction. In vertical-scanning, measurements are made by varying the distance Fxe2x80x2, either by moving the objective 10 or the sample S. Obviously, the OPD adjustment is limited by the subjective nature of human measurements and by the relatively coarse ability of the human eye to discern degrees of contrast in an interferogram. This is particularly true in the case of nulled fringes, where a single, uniform fringe covers the entire field-of-view.
As interference microscopes have become standard quality-control tools in production environments, such as for testing the topography of magnetic heads, greater measurement precision and repeatability are required. Such interference microscopes are now capable of making measurements with sub-nanometer precision. Accordingly, the precise minimization of the OPD between the reference mirror and the test surface has become more and more critical. This is especially true with objectives of high numerical aperture and correspondingly short depth of focus. Even though it is possible to maximize the fringe contrast (minimum OPD) in the Linnik interferometer at a condition where the reference mirror is out of focus, this condition is known to degrade the measurement performance of interference microscopes, as detailed in U.S. Pat. No. 5,978,086.
It has therefore become necessary to obtain extremely tight levels of control over the length of the reference path with respect to the test path, encompassing distances D and F in FIG. 1. In addition, it would be very desirable to have an automated method and apparatus for minimizing the OPD in white-light interference objectives in order to avoid the subjectivity and lack of precision of human measurements. The present invention is directed at improving the quality of the fringes generated by the interference between the reference and test beams by ensuring that the OPD between the two beams is optimally minimized. Two solutions are provided for optimizing the objective prior to performing phase-shifting or vertical-scanning interferometry. One solution is provided by maximizing the fringe contrast in the process of OPD minimization. It is suitable for general test surfaces, but requires separate optimization of reference mirror focus. The second solution entails maximizing the fringe modulation across the test surface. While it is suitable for a limited number of test surfaces with specific attributes, it allows for the simultaneous optimization of reference mirror focus and OPD, and can be employed in the critical measurement of magnetic heads.
One primary object of this invention is a method for optimizing the process of minimization of the optical path difference between the reference and test beams of a white-light Linnik interferometer.
Another primary objective of the invention is a method for setting the in-focus position of the reference surface of an interferometric microscope objective.
Another object of the invention is a method that is suitable for automated implementation.
Still another objective is a method and corresponding implementing apparatus that are suitable for automated use while the interferometer is in service for quality control in an assembly line.
Another goal of the invention is a method and apparatus that are suitable for incorporation within existing instruments.
A final object is a procedure that can be implemented easily and economically according to the above stated criteria.
Therefore, according to these and other objectives, the present invention consists of incorporating a mechanism for producing relatively large translations of the reference optics and mirror in a white-light Linnik interference objective and carrying out PSI measurements while the OPD between the reference and test beams is varied. Utilizing the same algorithms used in the art to perform conventional phase shifting interferometry, the position of minimum OPD is determined by finding the corresponding position of maximum intensity contrast of the interference fringes resulting from the superposition of the reference and test beams as the OPD is varied. Alternatively, for a test surface with specific characteristics, such as a magnetic head air bearing surface, the OPD is minimized and the reference mirror is simultaneously focused by maximizing the variation in fringe modulation across the test surface. By automating the system with a precise translation mechanism, operator-to-operator variations are completely eliminated and the precision of the process of OPD minimization and reference mirror focus are greatly improved.